1. Field
The invention relates to the recording and reproduction of digital audio, and particularly to an improved digital audio format, and method and apparatus thereof, for digitally recording and retrieving audio signals with error correction and concealment techniques.
2. Prior Art
As commonly known, the use of digital techniques has spread rapidly due to the ease with which digital data can be manipulated, transferred, transmitted and stored. Accordingly, as has happened in various technical fields such as the computer, instrumentation and video recording fields, digital techniques have recently acquired significant potential in the field of recording and reproducing of audio signals.
The object of any recording process is to store information and then faithfully reproduce it. However, in conventional analog recorders there exists a number of problems which deteriorate faithful reproduction, which problems are an inherent function of the recording medium and of the mechanical apparatus used to transport the medium. Although the problems have been compensated, or circumvented, by the development of very sophisticated mediums and mechanics, it is widely recognized that conventional analog recording/reproducing techniques are rapidly approaching theoretical operational limits.
Typical of problems encountered in analog recording/reproducing techniques are inadequate dynamic range, i.e., low signal-to-noise ratio, inherent phase distortion, inherent harmonic distortion, insufficient transient response, modulation noise, cross talk, print through, multi-copy degradation, flutter and wow, inherent limitations in noise reduction systems, storage degradation with time and limited low-end frequency response.
On the other hand, digital recording/reproducing techniques provide either an improvement in, or total elimination of, each of the above problems. Some of the problems, such as modulation noise, print through, inadequate dynamic range, harmonic distortion, modulation noise and low end limitations, are eliminated or significantly improved due to the fact that the problem does not exist in the digital domain. Other problems, such as phase distortion, transient response, flutter and wow, and storage and multi-copy degradation, are eliminated or significantly improved due to the ease with which the signal can be handled once it is converted to the digital domain.
However, the use of digital audio techniques in turn gives rise to various problems and disadvantages. For example, poor transmission conditions that conventionally would only degrade an analog signal may completely destroy the equivalent digital signal, and even a small discontinuity such as a single bit error, may cause serious audio degradation and unpleasant sounds if the bit error occurs at a significant bit position. That is, digital signal systems characteristically fail abruptly, usually without the gradual warning which is typical of deterioration in analog systems. Thus, it has been found that digital audio techniques require the use of special error correction, concealment and/or muting techniques to minimize the effects of the various types of dropouts and data errors arising during the reproduction of the recorded digital audio.
In order to effect efficient correction and/or concealment of errors, it is first necessary to detect that an error has occurred. A first level of error indication is provided by observing the playback RF signal envelope. However, such a technique fails to provide the requisite degree of detail required for a reliable error detection system.
Thus, in a high performance digital audio system, an optimum error detection technique includes the process of recording additional information along with the normal audio signal data. This information, termed "overhead", may be in the form of parity bits and/or special error checking characters, which are capable of providing detection of any error which may occur during the record or playback processes.
Upon detection, the errors may be either concealed and/or corrected. Concealment techniques may employ a zero order interpolation concealment where the last accurate data sample is held, or a first order interpolation concealment where an interpolation is made between the last accurate data sample and the next occurring accurate data sample.
The most desirable technique for eliminating errors is to correct them. This requires knowledge of the data recorded during the time that the error occurred. Thus, error correction techniques require the addition of the overhead information of previous mention during the recording process. Since errors generally are not randomly scattered but exist in bursts lasting from a few to several hundred bits, the error correction information must be dispersed and recorded on the recording medium to prevent the burst type errors from precluding precise operation of the error correction system. Thus, it follows that the more effectively and reliably that an error concealment and correction technique is, the more overhead information must be added to the audio data during recording. This additional overhead increases the data storage requirements of the recorder and either increases the packing density on the medium or causes a corresponding undesirable increase in tape speed and usage.
Thus, the method and format used to intersperse the overhead information with audio data is important in providing error concealment or correction of a gradual deterioration of the recorded data bit stream while precluding the total failure of the correction and thus of the digital audio recorder/reproducer system.
A fairly comprehensive list of articles on digital audio recorder/reproducer systems is compiled in the list of references and bibliography of an article by M. Willcocks entitled "A Review of Digital Audio Techniques", Journal of The AES, Jan-Feb, 1978, Volume 26, pages 56-64. Typical of such prior art are the systems described in Bellis & Brookhart AES preprint no. 1298 (M-2) Nov. 4-7, 1977; BBC Research Department report, Bellis and Smith BBC RD 1974/39, Nov. 1974; N. Sato, "PCM Recorder, A New Type of Audio Magnetic Tape Recorder", Journal AES, V. 21, No. 7, Sept. 1973; U.S. Pat. No. 3,930,234 to Queisser, et al; U.S. Pat. No. 3,994,014 to S. G. Burgiss.